Electrical inverters may be used for transforming an input voltage into an output AC voltage. For example, the AC voltage from an electrical grid may be transformed into a variable AC voltage supplied to an electrical drive or another AC voltage to be supplied to another electrical grid.
For generating the usual multi-phase output voltage, the inverter can include a plurality of semiconductor switches, for example thyristors or IGCTs, which may be controlled by an electronic controller of the inverter.
One possibility of controlling an inverter is direct torque control (DTC). In DTC the torque and the flux of the electrical drive may be controlled by estimating the actual torque and the actual flux form measured voltages and currents that are output from the inverter, and selecting a switching state for the switches of the inverter in such a way that the actual flux and actual torque move towards a reference flux and a reference torque, when the switching state is applied to the inverter switches.
However, one of the drawbacks of DTC may be the fact that the average switching frequency of the inverter cannot be directly controlled. Since the switching frequency can be proportionally related to the switching losses of the inverter, which may be a major part of the overall losses of the electrical drive, any reduction of the switching frequency may have a significant impact on the operational cost of the drive and increase the overall system robustness and reliability.
Such a reduction has been shown to be possible through the Model Predictive Direct Torque Control (MPDTC) method, which, for example, is described in EP 1 670 135 A1. In MPDTC, possible sequences of switching states are determined, which may be applied to the inverter switches in the future. These switching sequences may be constrained by the actual switching state of the inverter and the inverter topology. Since two different switching states may result in the same output phase voltages (i.e., the same voltage vector) possible voltage vector sequences may be considered that include a sequence of voltage vectors. For each possible switching sequences or voltage vector sequence, the switching losses are estimated and the first switching state of the switching sequence is applied to the motor.
For achieving a more drive friendly voltage supply, it is possible to place a harmonic filter, for example an LC filter, between the inverter and the electrical drive. Such a filter may smooth out the effects of irregular switching actions of DTC, which may lower the harmonic distortion of the motor currents.
However, in this case, the introduction of the harmonic filter may render drive quantities, like flux and torque, not directly controllable. Specifically, it is no longer possible to directly and rapidly manipulate the stator flux by the application of a specific voltage vector, since what is applied to the motor terminals is the voltage of the capacitor of the LC filter, which features much slower dynamics, and is not immediately affected by the applied voltage vector. This implies that the control objective of producing the desired torque by the suitable and rapid positioning of the stator flux vector is no longer attainable.
To address this issue with DTC, one solution is to modify the control problem and target the control of certain inverter (instead of motor) variables. Namely, the notions of the inverter flux and inverter torque are introduced; the first being the integral over time of the inverter voltage, and the second expressing the interaction of the inverter flux and the inverter currents. These two variables are different from the actual corresponding motor flux and torque (especially during transients) but their average values at steady state are the same. Those virtual notions are the electric equivalent of the motor torque and flux though they do not correspond to physical quantities. The advantage of introducing and working with such quantities is that they can be directly and quickly manipulated by the application of the proper voltage vector. These features imply that the DTC problem can be recast as an inverter control problem, where the objective is to keep the inverter flux and torque within certain bounds. The physical properties of the system then assure that the motor will also reach the appropriate steady state conditions.